Chalcogenide glasses are optical materials that have great ability to transmit IR radiation and which exhibit high chemical stability and heat resistance. By virtue of these characteristics, chalcogenide glasses have been extensively used as materials for making IR radiation transmitting windows and filters. If chalcogenide glasses could be drawn into fibers, they would be suitable for use not only as information transmitting lightguides of the type that has already been realized with silica glass fibers but also as waveguides for transmission of energy (such as those generated from CO and CO.sub.2 lasers) and radiation thermometers.
Chalcogenide glasses are glasses containing as a main component(s) sulfur (S), selenium (Se) and/or tellurium (Te), and their IR radiation transmitting range is shifted to the longer wavelength side by increasing the concentration of Be which has the largest atomic weight. If chalcogenide glass fibers are to be used as waveguides for energy transmission or radiation thermometers, it is preferable for the Te concentration to be increased. However, as the Te concentration increases, the glass becomes more unstable against crystallization, thereby making it difficult to draw fibers from the glass. Furthermore, chalcogenide glasses are highly reactive with oxygen, which will cause strong absorption in the wavelength range of 6 to 14 .mu.m. Therefore, chalcogenide glasses, especially those containing large amounts of Te, must be drawn into fibers at the lowest possible temperature and within an oxygen-free inert atmosphere.
It is generally desirable for optical fibers to have a core-cladding structure in which the core material is surrounded with a certain thickness of cladding material having a lower refractive index than the core material. This core-cladding structure is preferred not only for the purpose of reducing the transmission loss of the fiber but also for improving its mechanical strength and weatherability. Arai et al. conducted an experiment involving transmitting the power of a CO laser through As.sub.2 S.sub.3 glass fiber with a Teflon coating as a cladding material and they reported the occurrence of an increased transmission loss in the wavelength range of 2 .mu.m or longer on account of absorption by Teflon coating (T. Arai, M. Kikuchi, S. Sakuragi, M. Saito and M. Takizawa; Proc. of SPIE, 576 (1985) 24). It is, therefore, desirable for the chalcogenide glass fiber to have a cladding that is made of a material having no absorption in the IR range, preferably a chalcogenide glass.
While several methods for drawing chalcogenide glasses into fibers have been reported, none of them have yet reached the commercial stage. One method consists of melting a chalcogenide glass in a crucible with a nozzle in its bottom portion and drawing a fiber through the nozzle with the interior of the crucible being pressurized with argon gas (T. Katsuyama, K. Ishida, S. Satho, and H. Matumura; Appl. Phys. Lett., 45 (1984) 925, and N. J. Pitt, G. S. Sapsford, T. V. Clapp, R. Worthington and M. G. Scott; Proc. of SPIE, 618 (1986) 124). This approach is effective in retarding the crystallization of glass and maintaining an inert atmosphere, and GeSe and GeAsSe glass fibers have been prepared by this method. This method, however, has two main disadvantages; it is unable to produce a fiber having a core-cladding structure, and it is difficult to perform continuous drawing operations for an extended period because all of the chalcogenide glass in the crucible is uniformly heated and this sometimes leads to devitrification of the residual chalcogenide glass in the crucible during the drawing operation. P. Klocek et al. prepared a Ge-Sb-Se fiber having a core-cladding structure by the rod-in-tube preform method which is commonly employed to manufacture silica glass fibers (P. Klocek, M. Roth, and R. D. Rock; Opt. Eng., 26 (1987) 88). Their method consists of inserting a cylindrically worked core rod into a cladding tube which is also cylindrical in shape and drawing them simultaneously into a fiber. However, the resulting fiber shows a strong absorption peak due to the presence of oxide at a wavelength of about 8 .mu.m and it also suffers a high transmission loss (.gtoreq.5 dB/m) in the wavelength range of 3 to 11 .mu.m. It is probable that these problems are not only caused by the oxidation of the surface of the chalcogenide glass during drawing but also by light scattering due to structural imperfections at the core-cladding interface.